There's the Weird Al way of making funny music: steal a melody, add zany high-concept lyrics and rock on.

It takes more than that, though, to craft an original song from scratch that's not only funny and enjoyable to the ear, but also fits snugly into a movie plot. And in this season of witty movie music, it takes even more to stand out.

Alvin and the Chipmunks begin their new feature with a laughably sincere rendition of Daniel Powter's "Bad Day" — better known as the 2006 "American Idol" you're-voted-off song. In Tim Burton's upcoming adaptation of the musical "Sweeney Todd," Johnny Depp and Helena Bonham Carter peer out from a meat pie shop, trading barbs over potential ingredients in the dark chuckler "A Little Priest."

Producer and co-writer Judd Apatow's biopic parody "Walk Hard," opening Friday, features the most ambitious set of big screen comedy tunes to come along since Christopher Guest's 2003 folk revival romp "A Mighty Wind."

The original music of "Walk Hard" spans decades of pop styles, from the innocent '50s ("Take My Hand") to Ghostface Killah (on stage with Jackson Browne, Jewel and Lyle Lovett in a tribute version of "Walk Hard").

"There was a lot of debate about how funny the songs should be," Apatow said. "We wanted the songs to be good and to feel like he had all these hit records. But we wanted them to be also weird and wrong. It was a trick — writing a good bad song."

To get there, director and co-writer Jake Kasdan recruited songwriter friends Dan Bern, Mike Viola and Charlie Wadhams to add double entendres, traditional melodies and a dose of political incorrectness.

The first step was landing John C. Reilly to star as Dewey Cox, whose none-too-subtle character name is actually one of the most subtle phallic jokes in the film. Reilly shares Apatow and Kasdan's bent sense of humor, and also has the authentic singing chops to pull off a cross-country concert tour promoting "Walk Hard."

"He is the kind of actor who would actually get cast in one of these movies," Kasdan said of the Oscar- and Tony-nominated Reilly. "It just so happens that nobody had made one yet where he looks like the person they were depicting."

Reilly said he had always carried a guitar with him while shooting movies, and was happy to bring his passion for music into his more lucrative Hollywood career. "It was more like a hobby for a long time," Reilly said. "Now it's become part of my work."

His ad-libbing during an early recording of the title song helped set the tone for other tunes: "Folks used to tell me, 'Slow down Dewey. Why you gotta walk so hard?' I used to tell them life's a race. And I'm in it to win it, and I'll walk as hard as I please."

"That just started to inform the character of the music, which is this idiot badass," Kasdan said.

Writing from that perspective allowed Bern to stop self-editing when a silly idea came along. He wrote or co-wrote more than 100 Cox songs over the past 1 1/2 years, stopping only last week after rejiggering "Walk Hard" to "Block Hard" for a "Monday Night Football" promo.

Bern injected a bit of Woody Guthrie, a bit of Bob Dylan, and a lot of naive condescension into Cox protest songs like "Let Me Hold You (Little Man)," honoring little people, and "Dear Mr. President," which begins with the line, "Dear Mr. President, I want you to know I am deeper than you."

Humor is "just one of the colors that I play with," said Bern, who wrote a piece called "Talkin' Al Kida Blues" a few months after the Sept. 11 attacks. "It's a really effective way of getting things across that otherwise people might be resistant to," he said. "If they laugh, that's a way in."

Another of Bern's Dylanesque Cox tunes, "Royal Jelly," goes to gibberish as Reilly sings, "My sense of taste is wasted on the phosphorous and orange peels of San Francisco ax-encrusted frenzy. So let me touch you."

Songwriter Marshall Crenshaw's infinitely more subtle "Walk Hard" finds its humor not as much in the lyrics but in its phrasing, as Reilly repeats and stretches out the word "hard." He wrote the opening riff before any of the lyrics.

"It's sort of like powerful and ridiculous at the same time," Crenshaw said. "My emphasis wasn't really on jokes, it was on really having the song come across strongly."

That's the approach taken by innovators in modern funny songs, Stan Freberg and Tom Lehrer, said Harry Shearer.

"The idea is never to make the music funny. It's not Spike Jones time," said Shearer. He co-wrote the Guest classic "This Is Spinal Tap" (by which all musical biopic parodies must be judged), and just earned his first Grammy nomination for best comedy album, for "Songs Pointed and Pointless."

"It's always making it musically credible and having your fun on top of that," he said.

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